For osmotic pre-treatment, higher draw solution osmotic potential substantially increases the system's operating window of feed concentration. Conventional systems typically have a limit on draw solution concentration for reverse osmosis (RO).
In RO systems, the maximum feed salinity treatable is limited by the maximum salinity of the draw solution. The draw solution is typically limited to a high point of 80,000 ppm, as any greater concentration would require a RO hydrostatic pressure that would rupture the membrane. Maximum RO operation hydrostatic pressure is typically between 1000 and 1200 psi. The following generally explains this limitation. The flux across the RO membrane is proportional to the active membrane pressure. The active membrane pressure is the difference in hydrostatic pressure across the RO membrane (typically 800 psi feed to near atmosphere permeate) less the difference in osmotic pressure across the RO membrane (typically 500 psi feed to near 0 psi permeate). The active membrane pressure may be adjusted by changing any of the aforementioned values. While the osmotic pressure of the feed is easily adjustable in the system, the osmotic pressure of the permeate is typically fixed near zero, and is a function of the rejection of the RO membrane (defined as one minus the fraction of salt that passes into the permeate, typical RO rejections are greater than 99%).
Consequently, purification systems that are capable of treating feed solutions in excess concentrations of 80,000 ppm typically use heat and phase change, resulting in large foot prints, high energy demands and high capital system costs compared to membrane driven systems. These systems are used when the feed total dissolved solute (TDS) is greater than 80,000 ppm, or when lower TDS feed solutions must be treated to high recoveries with rejects exceeding 80,000 ppm (e.g., zero liquid discharge applications).